1. Field of the Invention
The invention resides in the field of electrolytic devices and more particularly relates to chlor-alkali or alkali metal chloride cells containing cation selective membranes.
2. Description of the Prior Art
The electrolysis of alkali metal chlorides with cation selective membranes for the production of chlorine, alkali hydroxides, hydrochloric acid and alkali hypochlorites is well known and extensively used, particularly with respect to the conversion of sodium chloride. In the sodium chloride process the electrolysis cell is divided into anolyte and catholyte compartments by a permselective cation membrane. Brine is fed to the anolyte compartment and water to the catholyte compartment. A voltage impressed across the cell electrodes causes the migration of sodium ions through the membrane into the catholyte compartment where they combine with hydroxide ions formed from the splitting of water at the cathode to form sodium hydroxide (caustic soda). Hydrogen gas is formed at the cathode and chlorine gas at the anode. The caustic, hydrogen and chlorine may subsequently be converted to other products such as sodium hypochlorite or hydrochloric acid.
The efficiency of these cells for production of caustic and chlorine depends upon how they are operated, that is, the balancing of the chemical parameters of the cell and the internal use of the products and further how the cells are contructed, i.e., what materials are used to form the components and what system flow paths are employed.
One particular concern in attaining efficiency is the control of the pH of the anolyte compartment. It is desirable to maintain the level as acidic as is necessary and sufficient to inhibit the formation of sodium chlorate and/or oxygen in the anolyte particularly where a recirculating brine feed is employed. Sodium chlorate and/or oxygen are formed when hydroxyl ions migrate from the catholyte compartment through the membrane into the anolyte compartment. Adding acid to the anolyte compartment neutralizes the hydroxyl ions and inhibits chlorate build up and oxygen evolution in a recirculating system. Such a procedure has been described in U.S. Pat. No. 3,948,737, Cook, Jr., et al, and elsewhere.
It has been recognized that the use of fuel cell type spaced porous catalytic electrodes with a surplus of available fuel may be advantageously employed in electrochemical cells of the type described for the purpose of reducing the external energy requirements of the cell. The fuel cell reaction supplies a portion of the electrical energy and reduces in part the necessity for supplying external energy for the formation of gaseous products. This concept has been extensively examined in U.S. Pat. No. 3,124,520, Juda. The product of the cell is hydrochloric acid rather than chlorine.
In that patent, the use of gas electrodes in a chlor-alkali type cell is described. The anode is composed of a water-proofed, porous conductor capable of activating a surplus of a combustible fuel such as hydrogen gas. An aqueous solution of sodium chloride or brine forming an anolyte is introduced into the anode compartment. The porous fuel anode functions as an agent for releasing into the anolyte hydrogen ions which in conjunction with the chloride ions supplied by the sodium chloride form hydrochloric acid. The latter is then withdrawn from the cell. Substantial amounts of chlorine gas are not formed. The hydrogen supplied to the anode may be obtained from the cathode where hydrogen is formed as a result of the electrolytic breakdown of water in the cathode compartment.
The present invention comprises an improvement over the above discussed prior art techniques particularly as applied to large volume production chlor-alkali cell apparatus where conservation of energy and utilization of process products and raw materials are important considerations in the economic feasibility of such units. In the method of the invention, this is accomplished by measuring the pH of the anolyte, passing a controlled substoichiometric amount of hydrogen to a spaced porous catalytic anode and controlling the pH of the effluent from the anolyte to the range of 2 to 4 by controlling the rate of hydrogen feed, thereby maximizing the efficiency of the cell. The advantages and features of the improvement will become apparent from the following summary.